Method of purifying metal



Dec. 16, 1947. 1 RHBRACE 2,432,856

METHOD OF PURIFYING METAL Filed June 9, 1945 CULLECTING MfMBE/Z. FOB OXIDE, SULPHIDE A/YD PHOSPH/DE DUST "TEAP Fo/a CH4] co A/VD co 645E003 HTMUSPHEEE ca/vm/////ve OXIDES, --/0 JULPH/DES ETC,

-'//YDUC TOR COIL METAL T0 .86 FUR/F150 2 era/e 5 j/ man /8 COOL [1Y6 IVA TEE. EXIT VACUUM PUMP I To Z INLET/1 .26) ATMOSPHL'KE 7 WITNESSES: INVENTOR M Porzer l7. Brace.

Patented Dec. 16, 1947 UNITED STA'l' ran METHOD OF PURIFYHNG METAL Application June 9, 1945, Serial No. 598,474

Claims. 1

This invention relates to the metals.

It has been recognized that the physical properties of metals in general may be considerably improved if the metals are freed of non-metallic contaminations. For example, it is known that the green oxide of chromium, CrzOs is very stable and, as such oxide is slightly soluble in chromium, it has an adverse efiect on the ductility of chromium. Other metals are also adversely affected by similar oxides and other non-metallic impurires.

Heretofore attempts have been made to free the difierent metals of oxides present either as inclusions or in solution. It is known that hydrogen will reduce oxides but it has been impractical heretofore to reduce the lower oxides of some of the metals and alloys in that it has been impractical to produce hydrogen suiiiciently free of oxypurification of I gen-bearing gases in large enough volumes by conventional methods to enable the efifective reduction of and removal of oxides from the metal.

An object of this invention is to provide a method of purifying metals.

Another object of this invention is to provide for reducing the oxides of metals while at the same time producing hydrogen free of oxygenbearing gases.

Other objects of this invention will become apparent from the following description when taken in conjunction with the accompanying drawing, the single figure of which is a View in section illustrating apparatus for practicingthe method of this invention.

In general, it has been found that only those metals having a vaporization rate of not less than 4X10- grams per square centimeter per hour when heated at a temperature between 70% and 95% of the melting temperature of the metal can be successfully treated in accordance with the teachings of this invention to provide for the removal of the smaller residues of impurities such as oxygen, sulphur and phosphorus. Metal selected'from the group. consisting of titanium, zirconium, vanadium, chromium, manganese, copper, cobalt, nickel, iron, columbium. tungsten, molybdenum and hafnium and alloys thereof can be successfully treated as described hereinafter to purify the metal.

All of the metals referred to and the alloys of such metals, for example 18-8 stainless steel, molybdenum base alloys containing to 20% tungsten with or without .05% to .5% chromium or other well known combinations of the alloy ing components, have melting. points in excess of 2 1090" C. and when heated sublimate or vaporize at rates depending upon the temperature at which they are heated, For example, tungsten, which has the lowest vaporization rate of the metals listed, has too low a vaporization rate when heated at a temperature of 2360 C. to be readily purified but when heated at a temperature of 3060" C. or approximately of its melting temperature is found to have a vaporization rate of l 10 grams per square centimeter per hour and to be readily purified by the method to be described.

Referring to the drawing, this invention is illustrated by reference to a representative apparatus utilized in effecting the purification of metals as contemplated by this invention. As illustrated, the apparatus comprises a bell l'ar it of suitable material, such as glass, inverted to seat on a sealing washer l2 disposed on a base member id. The base member I4 is provided with suitable apertures for receiving in an insulated manner, the leads l6 and H] of an inductor coil to and conduits 22 and 2 3, The inductor coil is of the usual type being formed of copper tubing through which a cooling medium, such as water (not shown) is disposed to be circulated, the leads l6 and 18 being connected to a suitable source of alternating-current power for producing an alternating magnetic field within the confines or" the coil.

The conduit 24 has a valve 25 connected therein and is disposed to be connected to a vacuum pump 25 and from thence through a valve 21 to the atmosphere for evacuating the chamber formed by the bell jar H] and the base id, or through a valve 29 to a storage tank 3!, as will be described hereinafter. Similarly, the conduit 22 is provided with a valve 28 and is disposed to be connected therethrough to a source (not shown) of hydrogen, the purpose of which will be explained more fully hereinafter.

Positioned adjacent the upper end of the inverted bell jar I9 is a cylindrical member so disposed substantially in alignment with the inductor coil 26 and maintained in its operative position by means of spring clips 32. The spring clips 32 are disposed in spaced relation about the outer wall of the cylindrical member 36 being secured thereto in any suitable manner, such as by welding, the outer edges of the spring clips 32 being spring biased into engagement with the inner walls of the bell jar Hi to maintain the cylindrical member 30 in its operative position relative to the inductor coil by a friction contact with the bell jar. If desired, suitable lugs 33 may be provided on the inner side walls of the bell jar In for the free ends of the spring clips 32 to seat against when the cylindrical member 30 is in its relative operating position.

As illustrated, the cylindrical member 3il is provided with an obstruction 35 disposed adiacent to and across its upper end. The obstruction 34 in the embodiment illustrated comprises two spaced wire mesh members and 38 between which is disposed a mass ii) of steel wool or the like, whereby the obstruction 3 3 comprises a labyrinthic collecting member being provided with a maze of passages therethrough, the purpose of which will be explained more fully hereinafter.

The cylindrical member is also disposed to carry a plurality of spaced lugs s2 disposed about its inner surface and secured thereto in any suitable manner, such as by welding for supporting a receptacle id within the inductor coil The receptacle l i preferably is formed 01 molybdenum in the form of a wire bird cage so as to form a substantially open receptacle of vertical wires joined only at the base as at .3. The receptacle M is maintained in its operative position in the inductor coil 26 by being hung from the lugs C2 by means of fine strands of molybdenum, the lower ends of which are looped over a ring as of ceramic material. The receptacle a l seats within the confines of the strands :28. With a charge 50 of the metal to be purified within the receptacle M, as illustrated, it is apparent that the metal 5i! may be readily heated by supplying power to the inductor coil 253.

In practicing the method of this invention, a charge 50 of metal to be purified, is disposed in the receptacle All and the bell iar it is positioned on the base it with the receptacle M disposed centrally oi"; the inductor coil 2!). The charge 50 may be any of the metals referred to hereinbefore or their alloys but for purpose of illustrating the invention, the method will be described as applied to a charge 59 of chromium. The chromium may be in any suitable form such as a bar, plate or slab or may be a block formed of compacted granules. Power is then supplied to the inductor coil 28 to heat the metal 58 to a temperature in the neighborhood of 700 C. or somewhat higher and with the valves 28 and 2'! closed, the valves 25 and 2? are opened to exhaust the bell jar. With the metal heated in this manner, it is found that the metal evolves sensible gases therefrom and where a vacuum of the order of.10- mm. of mercur is utilized, the chromium within the receptacle ts is quickly denuded of sensible gases.

The valve 26 is then closed and the valve opened to admit hydrogen to the bell jar Hi, the hydrogen preferably being as pure as can be obtained by conventional purification methods whereby the hydrogen contains not over 2% of oxygen-bearing gases. Sufiicient hydrogen is admitted to the bell jar ill until a pressure of not less than one cm. on the mercury scale is obtained and at the same time the power supplied to the inductor coil 20 is increased, whereby the metal is heated inductively to a temperature between '70% and 95% of the melting point of the metal to efiect vaporization of the metal 59 con-- tained in the receptacle M. In the case of chromium, it is found that a temperature of 1509 C. with a hydrogen pressure of approximately 1-0 cm. on the mercury scale is suilicient to efiect the required vaporization of chromium.

As the chromium vaporizes, the evaporated metal atoms merge with the hydrogen containing the oxygen-bearing gases as the gaseous atmosphere flows upwardly within the coil Ell. It is,

to circulate within the bell jar id in the direc- I tion illustrated by the arrows.

As the evaporated metal atoms merge with and are somewhat cooled by the hydrogen containing the oxygen-bearing gases, they react in accord- I ance with the formula 2Cr+3H2O 3I-I2+Cr2O3 to form the green oxide of chromium. The green oxide of chromium, CrzOa, appears as a fume or fog in the hydrogen atmosphere and rises with the hydrogen by convection into the cylindrical member 35 where the hydrogen flows to and through the labyrinthic collecting member 59. As the hydrogen containing the reaction product flows through the member w, the reaction products contact the collecting surfaces of the member such surfaces being cool relative to the zone of vaporization referred to hereinbefore, and the reaction products are deposited on the surfaces of the collecting member 40 in the form of particles of green dust. The hydrogen emerging from the collecting member Ml is free of the reaction products and flows downwardly, as indicated by the arrows between the outer surface of the cylindrical member 3i; and the inner surface of the bell jar iii to be returned to the lower end of the inductor coil 23), where it again rises by convection to flow through the zone of vaporization described hereinbefore.

As the hydrogen Hood of the reaction products flows upwardly within the coil 20, it envelopes the metal 58 in the receptacle t l and by contact with the metal, removes oxides therefrom in accordance with the formula The resultant oxygen bearing gases in the hydrogen thus formed, in turn, react with the evaporated metal atoms in the zone of vaporization to form the reaction products in the form of a fume or fog, as described hereinbefore, which by convection are carried to the collecting member ll) where the oxides are deposited on the collecting surfaces, thus completing the cycle.

As will be appreciated, by continuously circulating the hydrogen within the bell jar II) as described hereinb-efore, the oxides of chromium are progressively reduced and the reaction products are collected in the labyrinthic collecting member ll), whereby an extremely pure hydrogen atmosphere is obtained in addition to efiectlvely reducing the oxides of the chromium. As soon as the oxides of the metal 52 have been reduced in the manner described, the inductor coil 20 is disconnected from the power supply and the metal 50 is permitted to cool in the purified hydrogen atmosphere.

As a specific example, where bright annealed electrolytic chromium is treated in accordance with the method described, the oxide content of the electrolytic chromium is reduced from approximately 1%, which is standard for annealed electrolytic clean chromium, to .03%. This reduction has been obtained in a manner of 30 minutes or so with masses of chromium weighing approximately 1.5 kilograms. In another instance where a plate of electrolytic chromium was subjected to the method of this invention it has been found that the metal of the plate is so purified in a matter of -30;minutes "that the plate displays appreciable plasticity as compared 'to the extreme brittleness which in the past has characterized even annealed chromium plate. In all cases the surface of the .metal so treated is so clean that :it can readily be wetted'by c'opper at temperatures only .a few degrees higher than the melting point of copper. This is a striking demonstration of the absence of oxide films and, concomitantly, the high purity or the hydrogen attained :by :my :method.

'Wlii'le reference has been made so riar only :as 'to the removal of the oxides of the metal it will of course be appreciated that other impurities such .as sulphur and phosphorus when present .in the metal will react with the vaporized metal atoms and be deposited as sulp'l'iide dust and rphosphid'e dust in the labyrinthic collecting member.

stated 'herei-nbefore the vaporization tern .per-ature employed will depend on the metal or alloy which 'is to be purified. However, the vaporization or working temperature in :all cases is between 70% and 195% of the melting temperature, If the vaporization temperature is more than 95% of the :melting temperature, the metal being treated becomes so soft that it can not be successfully physically handled whereas if the temperature is below 70% of the melting temperature, the rate of vaporization is too slow to economically and emciently efiect the required purification of the hydrogen.

If the metal 50 being treated in accordance with this invention contains carbon, such impurity is also readily remove-d from the metal if a trap (not shown) containing charcoal, activated alumina or the like is positioned between the outer edge of the cylindrical member 30 and the inner surface of the bell jar Iii, so that as the hydrogen flows from the labyrinthic collecting member 10, the reaction products of carbon and hydrogen, such as CH4, CO and CO2 will be trapped therein. The trap 52 may be carried by the cylindrical member 40, being secured thereto in any suitable manner, such as by welding.

In all cases, it is found that the metals identifie-d hereinbefore and their alloys are readily purified when treated in accordance with the method of this invention, and that the hydrogen obtained after such purification of the metal is of itself extremely pure. It will be appreciated that the method of this invention can be practiced by utilizing a very small quantity of commercially purified hydrogen as the hydrogen is recirculated as described. If it is desired, the purified hydrogen may be removed from the bell jar 80 after the metal 50 is cooled by closing valve 27' and opening the valves 26 and 29 and evacuating the bell jar to a suitable container 3i for storing the purified hydrogen.

I claim as my invention:

1. In the method of purifying metal having a melting point over 1000 C. and a rate of vaporization of not less than 4x10 grams per square centimeter per hour at a temperature between 70% and 95% of the melting temperature of the metal, the steps comprising, positioning the metal in a closed chamber, evacuating the chamber while subjecting the metal to sufficient heat to efiect the evolution of sensible gases from the metal, admitting a hydrogen atmosphere containing oxygen-bearing gases to the evacuated chambenhea'ting the metal at a temperature between 70% end-% for the melting temperature of the metal to effect the vaporization thereof at a rate of not less than 4x10 grams per square centimeter per .hour, reacting the vaporized metal with the oxygen-bearing gases of the hydrogen atmosphere, passing the hydrogen atmosphere containing the reaction products to and through :a :labyrinthic collecting member to deposit the reaction products thereon, and returning the hydrogen atmosphere freed of the reaction products to the zone of vaporization of the metal to efiect the deoxidization of the metal.

2. .In the method [of purifying metal having a melting point over 1000 C. and a rate of vaporization of :not less than 4X10 grams per square centimeter per hour at a temperature between 70% and 95% of the melting temperature of the metal, the steps comprising, positioning the metal in a closed chamber, evacuating the chamber while subjecting the metal to sufficient heat to effect the evolution of sensible gases from the metal, admitting a hydrogen atmosphere containing oxygen-bearing gases to the evacuated chamber until a gaseous pressure of at least one centimeter of mercury is obtained, heating the metal at a temperature between 70% and 95% of the melting temperature of the metal to effect the vaporization thereof at a rate of not less than l 10- grams per square centimeter per hour, reacting the vaporized metal with the oxygen-bearing gases of the hydrogen atmosphere, passing the hydrogen atmosphere containing the reaction products to and through a labyrinthic collecting member to deposit the reaction products thereon, and returning the hydrogen atmosphere freed of the reaction products to the zone of vaporization of the metal to efiect the deoxiclization of the metal.

3. In the method of purifying metal having a melting point over 1000 C. and a rate of vaporization of not less than 4 l0 grams per square centimeter per hour at a temperature between 70% and 95% of the melting temperature of the metal, the steps comprising, positioning the metal in a closed chamber, evacuating the chamber while subjecting the metal to sufiicient heat to eliect the evolution of sensible gases from the metal, admitting a hydrogen atmosphere containing not over 2% of oxygen-bearing gases to the evacuated chamber, heating the metal at a temperature between 70% and 95% of the melting temperature of the metal to effect the vaporization thereof at a rate of not less than 4 l0- grams per square centimeter per hour, reacting the vaporized metal with the oxygenbearing gases of the hydrogen atmosphere, passing the hydrogen atmosphere containing the reaction products to and through a labyrinthic collecting member to deposit the reaction products thereon, and returning the hydrogen atmosphere freed of the reaction products to the zone of vaporization of the metal to effect the deoxidization of the metal.

4. In the method of purifying metal having a melting point over 1000 C. and a rate of vaporization of not less than 4X l0 grams per square centimeter per hour at a temperature between 70% and 95% of the melting temperature of the metal, the steps comprising, positioning the metal in a closed chamber, evacuating the chamber while subjecting the metal to sufiicient heat to effect the evolution of sensible gases from the metal, admitting a hydrogen atmosphere containing oxygen-bearing gases to the evacuated chamber, heating the metal at a temperature between 70% and 95% of the melting temperature of the metal to effect the vaporization thereof at a rate of not less than 4x grams per square centimeter per hour, reacting the vaporized metal with the oxygen-bearing gases of the hydrogen atmosphere, passing the hydrogen atmosphere containing the reaction products to and through a labyrinthic collecting member to deposit the reaction products thereon, recirculating the hydrogen atmosphere freed of the reaction products through the zone of vaporization where the purifled hydrogen combines with oxides to effect the deoxidization of the metal and the resulting oxygen-bearing hydrogen atmosphere reacts with the vaporized metal, continuing the circulation of the hydrogen atmosphere through the labyrinthic member and the zone of vaporization until the metal is purified, and then cooling the purified metal in the purified hydrogen atmosphere.

5. In the method of purifying metal having a melting point over 1000 C. and a rate of vaporization of not less than 4= 10 grams per square centimeter per hour at a temperature between 70% and 95% of the melting temperature of the metal, the steps comprisin positioning the metal in a closed chamber, evacuating the chamber 8 while subjecting the metal to suflicient heat to effeet the evolution of sensible gases from the metal, admitting a hydrogen atmosphere containing not over 2% of oxygen-bearing gases to the evacuated chamber until a gaseous pressure of at least one centimeter of mercury is obtained, heating the metal at a temperature between and of the melting temperature of the metal to efiect the vaporization thereof at a rate of not less than l 10- grams per square centimeter per hour, reacting the vaporized metal with the oxygenbearing gases of the hydrogen atmosphere, passing the hydrogen atmosphere containing the reaction products to and through a labyrinthic collecting member to deposit the reaction products thereon, recirculating the hydrogen atmosphere freed of the reaction products through the zone of vaporization where the purified hydrogen combines with oxides to effect the deoxidization of the metal and the resulting oxygen-bearing hydrogen atmosphere reacts with the vaporized metal, continuing the circulation of the hydrogen atmosphere through the labyrinthic member and the zone of vaporization until the metal is purified, and then cooling the purified metal in the purified hydrogen atmosphere.

PORTER H. BRACE. 

